Traveling-wave tubes (TWT) have been in existence for over forty years and are well known in the art. Traveling-wave tubes are comprised of an electron gun and collector positioned at opposite ends of a vacuum tube. The path of the electron beam, from the gun to the collector, is surrounded by a slow wave structure through which a RF wave is passed. The most basic structure, used in traveling-wave tubes, is a helix, wherein a wire is symmetrically wound around the path of the electron beam. The RF wave passing into the input of the helix has a known frequency. The velocity of the electron beam is adjusted in the traveling-wave tube so that the electron beam has approximately the same axial phase velocity as is present within the RF wave passing through the helix. The helix acts to slow the RF wave to a velocity reasonably obtainable by the electron beam. The longitudinal component of the electromagnetic field created by the slowed RF wave interacts with the electrons of the electron beam that have an approximate synchronism. The interaction between the electron beam and the slowed RF wave causes the electron beam to slow. The energy lost in the velocity of the electron beam, through the conservation of energy, produces an increase in the energy of the slow RF wave.
Obviously, the length and the number of windings of the helix surrounding the electron beam have a large effect on the performance of the TWT. Similarly, the acceleration potential, current and power of the electron beam also control the TWT's performance. In a TWT as the accelerating potential of the electron beam is reduced, the electron beam current must be proportionally increased to maintain the same electron beam power. The decrease voltage changes the frequency of operation of the TWT. In order to compensate for this change, the diameter of the surrounding helix must be decreased and the number of windings must be increased. Consequently, in order to maintain the same frequency of operation for the traveling-wave tube, a reduction of acceleration potential of the electron beam must be accompanied by a change in the size and shape of the helical windings.
Also, as the required frequency range increases above 40 GHz, the complexity of the fabrication of wide band helix TWTs is increased for reasonable accelerating potentials, as the frequency increases helix turns per inch increase, and helix diameter decreases.
The helix diameter and helix pitch of the traveling-wave tube circuit are limited by the present technology. Currently, the state of the art for miniature traveling-wave tube helical windings employs a 0.0025 inch diameter wire, wound around a 0.025 inch mandrel at a pitch of one hundred turns per inch. The technology to economically and efficiently reduce these dimensions further, in order to create low voltage designs for use with high current density electron beams and millimeter wave performance, is difficult and complicated. The invention can be employed in the frequency range of 18 GHz to 125 GHz, but once the frequency of operation exceeds beyond 40 GHz the present technology employing wire wound helices is extremely limiting.
The present invention eliminates the need for wire coil windings through the use of thick or thin film technology. By selectively placing segments of conductive material onto substrate layers and superimposing or stacking those substrate layers such that a segment of conductive material from one layer contacts the conductive segments of adjacent layers, a helix is formed that, by design, can be much smaller than conventional wire wound helical devices. The smaller dimensioned helix permits small traveling-wave tubes to be efficiently manufactured. With appropriate processing, the digital helix TWT can be incorporated into a monolithic design for use with integrated circuitry. The resultant tubes use very low voltage with high current density electron beams. Easily manufactured millimeter wave designs are also possible. Lower power amplifiers as a front end and some on chip power conditioning can be included on a multi-function hybrid or monolithic circuit. Digital phase and gain control of the TWT is also possible monolithically.
The creation of a helical structure employing substrate technology provides unique advantages over the prior art TWTs. While the prior art has employed multiple substrate layers to provide various structures, the prior art has not been directed to TWTs or the resultant problems in the miniaturization of TWTs. See U.S. Pat. No. 4,729,510 to Landis entitled COAXIAL SHIELDED HELICAL DELAY LINE AND PROCESS, issued Mar. 8, 1988, for a typical prior art structure using multiple substrate layers.
It is therefore an object of the present invention to provide a traveling-wave tube that has a unique helical structure surrounding the electron beam, which structure is formed by employing consecutive layers of thick or thin film substrates having predetermined conductive configurations deposited thereon.